The present invention relates to shoe presses for applying pressure to a running web of paper, paperboard, or the like. More particularly, the present invention relates to a shoe press of the type having a support which supports a press shoe adjacent to a cylindrical backing member such that the press shoe and backing member form an extended nip therebetween, and having a hydraulic device for urging the press shoe toward the backing member to apply pressure to the web running through the nip. The shoe press of the present invention can be used for example as a dewatering press in the press section of a paper or paperboard machine, as an extended nip calender or in a tissue machine where the backing member is formed by a Yankee dryer.
In a papermaking machine, a wet web of paper or the like from the forming section of the machine is typically carried through the nip of a shoe press of the above-described type, where the web is pressed between two layers of absorbent felt or the like for wicking moisture from the web. Such shoe presses can also be used for other purposes than dewatering. For example, a shoe press can be used as an extended nip calender. In an extended nip calender, the backing member may be a roll provided with heating means, e.g. an induction heater or internal channels for heated oil. When a shoe press is used in a tissue machine, the backing member may be a Yankee dryer. When a shoe press is used in the press section of a paper or paperboard machine to press water from a wet web, the backing member may be, for example, a deflection-compensated roll.
One of the difficulties encountered in shoe presses is thermal expansion of the shoe from frictional heating of the shoe by the belt that carries the paper web through the press, as well as from hot hydraulic fluid which is circulated through the shoe for various purposes. Thermal expansion of the shoe causes elongation of the shoe in the cross-machine direction. The shoe is typically urged by a hydraulic device toward the backing member. The hydraulic device may be formed by a pressure chamber which extends in the cross-machine direction the whole length of the shoe. The pressure chamber is limited by the press shoe at the radial outside of the chamber and a shoe bed beneath the shoe which rests on a stationary support member of the shoe press roll. Such a shoe press is disclosed in, for example, DE 44 02 595. However, the majority of the shoe presses that are currently produced are instead provided with a plurality of hydraulic cylinders that extend in the cross machine direction in one or several rows. In such a shoe press, the hydraulic cylinders must be able to allow thermal expansion of the shoe without damage to the cylinders and/or the shoe. Moreover, the operation of the shoe press must not be disturbed. In U.S. Pat, No. 6,083,352, a shoe press is disclosed that comprises a plurality of articulated hydraulic loading cylinders that are spaced apart in the cross-machine direction along the press shoe. Each loading cylinder includes a piston member disposed within a cylinder. One of the piston and cylinders comprises a two-piece member having a first member fixed relative to the press shoe and a second member fixed relative to the support and spaced from the first member. The other of the piston and cylinders comprises a coupler engaging both the first and second members. The coupler engages the respective first and second members at seals which enable the coupler to pivot relative to the first and second members about axes parallel to the machine direction. Thereby, the articulated hydraulic loading cylinders enable the press shoe to move in the cross-machine direction relative to the support.
For shoe presses, it can also be a problem that the hydraulic device or devices used for urging the press shoe toward a counter member can become pressurized even when no backing member is present. In U.S. Pat. No. 5,223,100, a shoe press is disclosed that has a shoe bed with a pressure chamber and a connecting element disposed between the shoe bed and a side of the press shoe. It is stated that the connecting element prevents the press shoe from unintentionally escaping from the shoe bed, for instance, under pressure prevailing in the pressure chamber or under the force of gravity. It is stated that the connecting element may include an auxiliary piston which is part of an additional cylinder-piston unit. A cylinder of the unit is fastened to the outside of the shoe bed. The auxiliary piston and the piston fastened to the shoe bed are described as forming a pair of stop surfaces that limits the stroke of the press shoe.
It is an object of the present invention to provide an improved shoe press with a plurality of hydraulic actuators spaced apart in the cross-machine direction that can accommodate thermal expansion of the shoe in the cross-machine direction and simultaneously allow pressurization of the hydraulic actuators even when no backing member is present. It is also an object of the invention to provide a shoe press that is easy to assemble and that allows easy access for service purposes. These and other objects of the invention are attained with the shoe press according to the present invention.